Search

Hydrogeologic systems in the southern Cascade Range in California (USA) develop in volcanic rocks where morphology, stratigraphy, extensional structures, and attendant basin geometry play a central role in groundwater flow paths, groundwater/surface-water interactions, and spring discharge locations. High-volume springs (greater than 3 m3/s) flow from basin-filling (

Subduction zones produce some of Earth’s most devastating geological events. Recent eruptions of Mount St. Helens and great earthquakes and tsunamis in Japan and Sumatra provide stark examples of the destructive power of subduction-related hazards. In the Cascadia subduction zone, large earthquakes, tsunamis, and volcanic eruptions have occurred in the past and geologic records imply that these events will occur in the future. As the population and infrastructure increase in the region, resilience to these natural hazards requires a detailed scientific understanding of the geologic forces and processes involved, combined with a society motivated to mitigate risks.

New mapping demonstrates that active emergent thrust faulting is occurring within the fold-and-thrust belt north of the deformation thrust front in the NW Himalaya. The >60-km-long Riasi fault system is the southeasternmost segment of a seismically active regional fault system that extends more than 200 km stepwise to the southeast from the Balakot-Bagh fault in Pakistan into northwestern India. Two fault strands, the Main Riasi and Frontal Riasi thrusts, dominate the fault system in the study area. The Main Riasi thrust places Precambrian Sirban Formation over folded unconsolidated Quaternary sediments and fluvial terraces. New age data and crosscutting relationships between the Main Riasi thrust and the Quaternary units demonstrate that the Main Riasi thrust accommodated shortening between 100 and 40 ka at rates of 6−7 mm/yr. Deformation shifted to the southern Frontal Riasi thrust splay after ca. 39 ka. Differential uplift of a 14−7 ka terrace yields a range of shortening rates between 3 and 6 mm/yr. Together, shortening across the two strands indicates that a 6−7 mm/yr shortening rate has characterized the Riasi fault system since 100 ka. Geodetic data indicate that an 11−12 mm/yr arc-normal shortening rate characterizes the interseismic strain accumulation across the plate boundary due to India-Tibet convergence. These data combined with rates of other active faults in the Kashmir Himalaya indicate that the Suruin-Mastgarh anticline at the thrust front accounts for the remainder 40%−50% of the convergence not taken up by the Riasi fault system. Active deformation, and therefore earthquake sources, include both internal faults such the Riasi fault system, as well as rupture of the basal decollement (the Main Himalayan thrust) to the thrust front. Limited paleoseismic data from the Riasi fault system, the historical earthquake record of the past 1000 yr, the high strain rates, and partitioning of slip between the Riasi fault system and the thrust front demonstrate that a substantial slip deficit characterizes both structures and highlights the presence of a regionally important seismic gap in the Kashmir Himalaya. Slip deficit, scaling relationships, and a scenario of rupture and slip on the basal decollement (the Main Himalayan thrust) parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw > 8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake on the Balakot-Bagh fault in Pakistan Azad Kashmir.

article i nfo Effects of climate change, retreating glaciers, and changing storm patterns on debris flow hazards concern managers in the Cascade Range (USA) and mountainous areas worldwide. During an intense rainstorm in November 2006, seven debris flows initiated from proglacial gullies of separate basins on the flanks of Mount Rainier. Gully heads at glacier termini and widespread failure of gully walls imply that overland flow was trans- formed into debris flow along gullies. We characterized gully change and morphology, and assessed spatial distributions of debris flows to infer the processes and conditions for debris flow initiation. Slopes at gully heads were greater than ~0.35 m m �1 (19°) and exhibited a significant negative relationship with drainage area. A break in slope-drainage area trends among debris flow gullies also occurs at ~0.35 m m �1 , representing a possible transition to fluvial sediment transport and erosion. An interpreted hybrid model of debris flow initi- ation involves bed failure near gully heads followed by sediment recruitment from gully walls along gully lengths. Estimates of sediment volume loss from gully walls demonstrate the importance of sediment inputs along gullies for increasing debris flow volumes. Basin comparisons revealed significantly steeper drainage networks and higher elevations in debris flow-producing than non-debris flow-producing proglacial areas. The high slopes and elevations of debris flow-producing proglacial areas reflect positive slope-elevation trends for the Mount Rainier volcano. Glacier extent therefore controls the slope distribution in proglacial areas, and thus potential for debris flow generation. As a result, debris flow activity may increase as glacier termini retreat onto slopes inclined at angles above debris flow initiation thresholds.

The upper reach of the Hutuo River flows along the Xin-Ding basin and cuts a transverse drainage through the Xizhou Mountain and the Taihang Range into the North China Plain. Previous studies showed that the Xin-Ding basin was occupied by a lake during the early-middle Pleistocene. However, the timing of the paleolake breach and the mechanism for the creation of the transverse drainage are unknown. We constructed the fluvial terrace sequence in the upper reach of the Hutuo drainage combined with thermoluminescence (TL) and optically stimulated luminescence (OSL) dating, as well as the timescale of the overlying loess–paleosol sequence. Our results reveal that (1) five terraces (T5–T1) developed along the upper reach of the Hutuo River, amongst which terraces T4–T1 were formed synchronously at ~ 600, ~ 120–130, ~ 20–26 and ~ 6–7 ka, respectively; (2) the creation of the transverse drainage and breach of the Xin-Ding paleolake occurred between ~ 600 and ~ 130 ka; and (3) the mechanism for the creation of the transverse drainage is via river piracy of paleostreams on both sides of the drainage divide. Localized differential uplift and associated tilting of the Xizhou Mountain block during the middle Pleistocene result in the formation of the transverse drainage and breach of the Xin-Ding paleolake.

Trenches excavated across the surface rupture of the 15 January 1944 M w 7 San Juan, Argentina, earthquake show evidence for repeated rupture of La Laja fault in the late Pleistocene and Holocene. The 1944 rupture extended for about 7 km striking ∼N35°E, and dipping 42° E parallel to Neogene bedding, with a maximum east‐side‐up vertical displacement of 30 cm. We interpret nine discrete colluvial wedges, each capped by a soil, as evidence for nine surface ruptures in the past ∼32–35 ka. Our topographic profile of the T3 terrace surface projected across the fault suggests these nine events collectively produced ∼10.3 m of dip displacement. We used radiocarbon and optically stimulated luminescence dating to define the timing of past events, yielding an average recurrence interval of 3.9±0.3 ka. The average estimate for displacement per event is about 1.1 m, compared with the 0.5 m of dip slip that occurred in 1944. Using a fault‐related fold model that infers ∼28 m of shortening based on the axial surface migration for the T 3 terrace, along with the ∼35 ka age of the terrace, yields a shortening rate of ∼0.8 mm/yr by the 1944 source thrust fault, which is about 20% of the Global Positioning System—constrained total shortening rate across the Precordillera. Using this shortening rate and an assumed fault dip of 30° implies a fault slip rate of about 1.1 mm/yr, which combined with the average recurrence interval, implies ∼4 m of average slip per event on the causative fault at depth. Online Material: Unit descriptions from the trenches, stepwise reconstruction from trench 1, and annotated photographs of the La Laja region.

A revolution in remote sensing, light detection and ranging (LiDAR) laser altimetry swath mapping, reveals the details of topographic features at such high resolution that they have transformed our understanding of tectonic forcing of the shape of the Earth’s surface. Meter-scale DEMs (digital elevation models) capture fault offsets, fault zone structure, off-fault deformation, and landscape properties at microgeomorphic scale, highlighting that the surface faithfully records the complexity and sensitivity of deformation in detail.

New data indicate that northeast-directed extensional faulting characterizes slip across the Brothers fault zone (BFZ), which marks the northern limit of the northwestern Basin and Range (NWBR) extensional province in southeastern Oregon. Structural separation across individual north-northeast striking NWBR faults decreases to zero south of the BFZ. Field relationships and cross-sections demonstrate limited kinematic linkage and independent evolution of the two fault systems since ∼7 Ma. West-directed extension accumulated on NWBR faults at 0.01 mm/yr and lengthened northward after 7.05 Ma. BFZ faults accumulated northeast-directed extension at rates of 0.01 mm/yr since 5.68 Ma. Deformation coincides with periods of heightened basaltic magmatism in the High Lava Plains, implying that volcanism weakened the crust and promoted extension in the BFZ. In a new model, we reconcile the observed northward diminishing rate and clockwise motion of the modern NWBR deformation field with regional geology. The BFZ defines a small circle about the pole of rotation and separates a stable block to the NE from the extending region to the south. Faults to the south are growing northward, consistent with the northward decrease in rate and magnitude of extension in the NWBR.

Comparison of geologic uplift patterns with results of three-dimensional mechanical models provides constraints on the fault geometry compiled by the South- ern California Earthquake Center community fault model in the northern Los Angeles basin, California. The modeled uplift matches well the geologic pattern of uplift as- sociated with the Santa Fe Springs and Coyote Hills segments of the Puente Hills thrust fault but does not match structures to the west of the San Gabriel River. To better match the geologic patterns in this area, alternative fault configurations were tested. The best match to geologic uplift is attained with a model incorporating (1) a steep blind thrust fault at the location of the Los Angeles segment of the Puente Hills thrust system (following interpretations of the Las Cienegas fault geometry at this location), (2) removal of an inferred linking fault between the Raymond and Holly- wood faults, and (3) lateral continuation of the Lower Elysian Park fault, a blind low- angle detachment at >10 km depth, along strike to the northwest. These geometric revisions alter the connectivity of northern Los Angeles basin faults and significantly improve the match of model uplift pattern to geologic data. Model results suggest that fault connectivity may be more important in governing fault slip rate than are fault dip and fault area. The preferred model alters slip rates by >0:2 mm=yr for the Upper Elysian Park, Hollywood, Lower Elysian Park, Raymond, Sierra Madre West, and Verdugo faults. Additionally, the preferred model alters the surface area of several faults in the northern Los Angeles basin, such as the Puente Hills thrust and the Lower Elysian Park fault, which may have important implications for seismic hazard assess- ment in the northern Los Angeles basin.

Glacial retreat and opening of Taan Fjord (an arm of Icy Bay, Alaska) in the last two decades drove a base level fall of ∼ 400 m at the outlets of four tributary valleys in the region of the current Tyndall Glacier terminus. Response in the tributary valleys to this base level fall includes evacuation of stored sediment, incision of slot gorges into bedrock, and landsliding on valley walls. Fluvial transfer of eroded bedrock, glacial deposits, and stored nonglacial fluvial deposits after 1983 resulted in progradation of fan deltas toward the fjord centerline at the outlet of each of the four catchments. In the largest nonglacial tributary valley, ∼ 0.08 km 3 (∼ 8%) of the 0.59 km 3 of > 500-m-thick stored fluvial and colluvial deposits has been transferred to the adjacent fjord. A percentage of the fjord sedimentation in the last two decades thus includes material eroded from bedrock and stored in the landscape during the previous glacial expansions. Distal deltaic deposits extend across the fjord floor and likely interfinger with distal proglacial deltaic deposits sourced from the Tyndall Glacier. A paraglacial landscape response, such as the one exhibited in upper Taan Fjord where sediment delivery to fjords from tributary sources in synchrony with sediment produced by primary bedrock erosion by glaciers, helps to explain the order-of-magnitude discrepancy between sediment yields from Alaskan tidewater glaciers on 10 0 - to 10 2 -year timescales and sediment yields and exhumation rates on 10 4 - to 10 6 -year timescales.

Relationships between riverbed morphology, concavity, rock type and rock uplift rate are examined to independently unravel the contribution of along-strike variations in lithology and rates of vertical deformation to the topographic relief of the Oregon coastal mountains. Lithologic control on river profile form is reflected by convexities and knickpoints in a number of longitudinal profiles and by general trends of concavity as a function of lithology. Volcanic and sedimentary rocks are the principal rock types underlying the northern Oregon Coast Ranges (between 46°30′ and 45°N) where mixed bedrock–alluvial channels dominate. Average concavity, θ, is 0·57 in this region. In the alluviated central Oregon Coast Ranges (between 45° and 44°N) values of concavity are, on average, the highest (θ = 0·82). South of 44°N, however, bedrock channels are common and θ = 0·73. Mixed bedrock–alluvial channels characterize rivers in the Klamath Mountains (from 43°N south; θ = 0·64). Rock uplift rates of ≥0·5 mm a−1, mixed bedrock–alluvial channels, and concavities of 0·53–0·70 occur within the northernmost Coast Ranges and Klamath Mountains. For rivers flowing over volcanic rocks θ = 0·53, and θ = 0·72 for reaches crossing sedimentary rocks. Whereas channel type and concavity generally co-vary with lithology along much of the range, rivers between 44·5° and 43°N do not follow these trends. Concavities are generally greater than 0·70, alluvial channels are common, and river profiles lack knickpoints between 44·5° and 44°N, despite the fact that lithology is arguably invariant. Moreover, rock uplift rates in this region vary from low, ≤0·5 mm a−1, to subsidence (

Spatial and temporal patterns of exhumation are inextricably linked to patterns of crustal deformation because crustal deformation drives rock uplift. A new interpretation of a segment of the Pacific-North America transpressional plate boundary in southern California is analyzed in the context of crustal shortening, rock uplift, and exhumation. Deformation is partitioned between two structural anticlinoria formed above thrust faults that root into a mid-crustal decollement. The southern anticlinorium began growing after 5 Ma and is characterized by almost no topographic expression, rock uplift of ∼3 km, and exhumation of

The interaction between tectonism and erosion produces rugged landscapes in actively deforming regions. In the northwestern Himalaya, the form of the landscape was found to be largely independent of exhumation rates, but regional trends in mean and modal elevations, hypsometry (frequency distribution of altitude), and slope distributions were correlated with the extent of glaciation. These observations imply that in mountain belts that intersect the snowline, glacial and periglacial processes place an upper limit on altitude, relief, and the development of topography irrespective of the rate of tectonic processes operating.

A six-step reconstruction of the South Pyrenean foreland fold-and-thrust belt in Spain delineates the topographic slope, basal dtcollement angie, internal deformation, and thrust- front advance from the Early Eocene until the end of contractional deformation in the Late Oligocene. Style of thrust- front advance, dip of the basal dtcollement, slope of the upper surface, and internal deformation are decoupled and not simply related. Internal deformation increased, decreased, and maintained surface slope angle at different stages. From the onset to the cessation of deformation, the basal dtcollement angle decreased overall suggesting translation of the thrust belt onto stronger crust with time. Taper angle of the Pyrenean thrust wedge was fundamentally controlled by the flexural rigidity of the lower plate, the relative rate of creation of structural relief in the rear versus the front of the wedge, the extent of deposition of eroded material within the deforming wedge, and the taper of the pretectonic stratigraphic wedge.

New mapping, structural analysis and magnetostratigraphy reveal a polyphase deformational history at the leading edge of the Spanish Pyrenean foreland fold-and-thrust belt. The present structural architecture is the consequence of three discrete periods of deformation. Map relationships between four syntectonic units and major structures provide tight geometric and age constraints for each deformational period (D1: 55 to 42 Ma; D2: 30.1 to 28.0 Ma; D3: 27.9 to < 25.8 Ma). Most of the shortening in the area accumulated during D2 thrusting at rates which varied from 0.7 mm y −1 to 2.8 mm y −1 along strike from northwest to southeast, respectively. Removal of D2 and D3 displacement on thrust faults restores folds formed during D1. A sequential kinematic history during which an individual structure accommodates shortening by folding, tightening of the fold and subsequent truncation by thrust faulting is inferred. Sequential restoration of three serial cross-sections demonstrates that nearly every structure active during D1 was also active during D2. Distributed deformation characterizes the temporal and spatial pattern on all time scales, from structures active during each individual deformational period to the long-term history encompassed by the total deformation.

The final, deformed state of a fold-and-thrust belt may be reached by an infinite number of kinematic paths. Two end-member kinematic paths are due to continuous or discontinuous rates of deformation. We have used a new magnetostratigraphic section from the Spanish Pyrenees to calibrate the emplacement history, over ≈10 m.y., of a major thrust sheet (the Sierras Marginales thrust sheet) and the deformation of both its hanging wall and footwall. Six time windows from before 36.5 Ma until after 24.7 Ma were recognized on the basis of structural and stratigraphic relationships between syntectonic strata and major structures. Footwall deformation of the Sierras Marginales thrust sheet occurred continuously on a detachment within the foreland-basin sequence (4.5 km net shortening; shortening rates steadily increased from 0.14 to 1.5 mm/yr with time). Although the detachments at the base of the foreland and the detachment at the base of the Sierras Marginales thrust sheet were active coevally, the rate of displacement on the Sierras Marginales detachment decayed with time and shows considerable variability. Emplacement of the thrust sheet may be divided into three distinct periods: a rapid 13.8 km translation from 37.0 to 36.5 Ma (27.6 mm/yr), a gradual climb of the toe of the thrust sheet up a 4.3-km-long ramp across the foreland-basin succession from 36.5 to 32.0 Ma (0.95 mm/yr), and a final 8.9 km translation from 32.0 to 29.5 Ma (3.56 mm/yr). Internal deformation of the thrust sheet occurred only after it reached its present position at some time before 29.5 Ma. Shortening rates steadily decreased from 0.6 mm/yr between 29.5 and 27.8 Ma to 0.26 mm/yr from 27.8 until after 24.7 Ma. Only ≈1 km of shortening accumulated during each of the two periods; the last shortening localized on the most northerly thrust in the study area. Folding and subsequent faulting above the detachment beneath the foreland suggest that slip was transmitted to its tip point continuously throughout the deformation. In contrast, translation followed by internal deformation on the hinterland side of the toe of the Sierras Marginales thrust sheet indicates a successive deactivation of the southern parts of the detachment with time. Rate of deformation on both detachments was discontinuous and shows substantial variability about the mean. In general, the spatial and temporal pattern of deformation was distributed and continuous. In detail, however, shortening was spatially and temporally discontinuous above each detachment, and structures related to each one display distinctly different deformational patterns, rates, and styles.

Responses of both modern and ancient fluvial depositional systems to growing folds can be interpreted in terms of interactions among competing controlling variables which can be incorporated into simple conceptual models. The ratio of the rate of sediment accumulation to the rate of structural uplift determines whether a fold develops a topographic expression above local base level. The balance between (a) stream power and rates of upstream deposition vs. (b) bedrock resistance and rates of crestal uplift and of fold widening determines whether an antecedent stream maintains its course or is defeated by a growing structure. Modern drainage configurations in actively folding landscapes can often be interpreted in terms of these competing variables, and through analysis of digital topography, detailed topographic characteristics of these folds can be quantified. Modern examples of growing folds display both defeated and persistent antecedent rivers, deflected drainages and laterally propagating structures. The topography associated with a defeated antecedent river at Wheeler Ridge, California, is consistent with a model in which defeat results from forced aggradation in the piggyback basin, without the need to vary discharge or uplift rate. Reconstruction of the long-term interplay between a depositional system and evolving folds requires a stratigraphic perspective, such as that provided by syntectonic strata which are directly juxtaposed with ancient folds and faults. Analysis of Palaeogene growth strata bounding the Catalan Coastal Ranges of NE Spain demonstrates the synchronous growth and the kinematic history of multiple folds and faults in the proximal foreland basin. Although dominated by transverse rivers which crossed fold crests, palaeovalleys, interfan lows, structural re-entrants and saddles, and rising anticlines diverted flow and influenced local deposition. In the ancient record, drainage-network events, such as avulsion or defeat of a transverse stream, usually cannot be unambiguously attributed to a single cause. Examination of ancient syntectonic strata from a geomorphological perspective, however, permits successive reconstructions of synorogenic topography, landscapes and depositional systems.

[1] Active mountain building associated with the accretion of the Yakutat microplate (YT) in southern Alaska is characterized through the combination of a new balanced cross section and new low-temperature cooling ages. This analysis constrains the amount and timing of shortening, the spatial and temporal trends of exhumation, and the interplay between structural development and exhumation. A fold-and-thrust belt comprising three principal thrust sheets (the Hope Creek, Sullivan, and an offshore thrust sheet from north to south) characterizes the YT internal structure, which has absorbed a minimum of ∼82 km shortening. Assuming shortening and foreland basin development occurred contemporaneously, the shortening rate across the YT is ∼13–14 mm/a after 5.6 Ma. Detrital apatite fission track ages, from south to north, are unreset along the southern edge of the Sullivan thrust sheet at the coast, are reset and have ages younger than 6.3 Ma within the thrust belt, and have reset cooling ages of ∼13 Ma in the North American upper plate. Only the zircon samples from the northern, internal YT are potentially reset. Exhumation rates within the thrust belt vary from 0.3 ± 0.1 mm/a to 4 ± 1.8 mm/a. Combining the thermochronometric and structural data indicate that wedge exhumation since ∼6 Ma is 500 km2 and that particle trajectories have larger horizontal than vertical components. Whereas exhumation and shortening have been focused on the windward side of the Chugach/St. Elias Range since ∼6 Ma, the cooling ages do not uniquely distinguish between orographically versus tectonically controlled erosion.

We document a new model of crustal structure of the Andean front in Argentina where numerous historic earthquakes destroyed the cities of Mendoza in 1861 (Ms = ∼7) and San Juan in 1944 (Mw = 7.0). The Cerro Salinas anticline is formed above the west directed Cerro Salinas thrust. An east facing monocline with an amplitude of about 2 km folds the Cerro Salinas thrust and overlying Neogene succession. This monocline is formed above a blind crustal thrust in the basement. Its dip of 14° west is inferred from fold geometry. This thick-skinned east directed blind thrust and the thin-skinned west directed Cerro Salinas thrust define a tectonic wedge; the wedge tip occurs at a depth of 5.4 km. Growth of the monocline after ∼8.5 Ma is revealed on multichannel seismic (MSC) profile 31017 (Repsol-YPF). Rates of Cerro Salinas thrust displacement are of the order of 1 mm/yr, whereas vertical uplift of ∼0.45 mm/yr results from the combined displacement on the Cerro Salinas thrust and growth of east facing monocline. The lateral extent of the east directed crustal blind ramp corresponds with the along-strike extent of the Eastern Precordillera. When combined with the low displacement rate, a long earthquake recurrence interval is implied. Smaller magnitude earthquakes, however, indicate that segments of the blind thrust ramps ruptured in historic events. If all the segments of the blind thrust ruptured together the fault area is ∼7000 km2 and could produce a Mw ∼ 7.7 earthquake. The crustal wedge model provides new constraints on the origin and potential size of earthquakes that threaten the densely populated region. Copyright 2007 by the American Geophysical Union., We thank Repsol-YPF S.A. for providing us with the seismic lines and for the permission to publish parts of them. This work has been partially supported by 99AR0010 CSIC-CONICET project and Grups de Recerca Consolidats (II Pla de Recerca de Catalunya) Projects 1997 SGR 00020. A.J.M. was supported by U.S. NSF grants EAR-0232603 and EAR-0409443 and a Research Equipment Reserve Fund grant from Oregon State University.

Active deformation in southern California occurs in two discrete structural provinces. To the south of ~ 34°N latitude, the Peninsular Ranges structural province is characterized by northwest-trending strike-slip faulting, northwest motion of crustal blocks relative to stable North America, and negligible vertical-axis block rotation. In contrast, the Transverse Ranges province to the north of ~ 34°N latitude is characterized by west-trending oblique-slip faulting and a significant component of north-south crustal shortening contemporaneous with northwestward motion and vertical-axis block rotation relative to stable North America. A structurally complex transition zone between N33°45' and N34°15' latitude defines the boundary separating these two provinces. Bedrock and geomorphic relationships in the eastern Santa Monica Mountains document the sequential development of part of this boundary region. At the longitude of downtown Los Angeles, the Malibu Coast- Santa Monica-Hollywood fault system and the Elysian Park fault systems have accommodated the bulk of the differential motion in the boundary zone between the two provinces over the last 2.5-5 m.y. Structural evolution of the boundary zone was dictated by the progressive increase in displacement and lateral propagation of each fault system, differential vertical-axis rotation of the Transverse Ranges block relative to the Peninsular Ranges block, and a decrease in the obliquity of convergence with time. This interaction transferred a portion of the Elysian Park anticline, an anticline developed in association with the Elysian Park fault system, to the hanging wall of the Hollywood fault. Net slip across the boundary zone north of downtown Los Angeles is limited to 1.7-2.5 km of dip-slip, to 1.5 km of strike-slip, and 2.3-2.9 km of oblique-slip during the past 2.5-5 m.y. These low values imply that the southern boundary of the Transverse Ranges has remained largely coupled to the adjacent Peninsular Ranges structural province during compression. Using a 5 Ma age for initiation of growth of the Santa Monica Mountains and Elysian Park anticlines enables minimum rates of 0.3-0.5 mm/yr, 0.3 mm/yr, and 0.5-0.6 mm/yr for the dip-slip, strike-slip, and oblique-slip components of motion, respectively. A 2.5 Ma age of initiation of the Elysian Park anticline would double these rates. A new tectonic model for the structural evolution of the boundary zone based on the sequence of structural overprinting reconciles apparently discrepant geologic, paleoseismic, and geodetic data, data suggesting that the boundary zone is dominated by long-term strike-slip, intermediate-term oblique-slip, and short-term dip-slip displacement.

An emerging paradigm that equates glaciers to ''buzz saws'' of exceptional erosional efficiency has been strengthened by short- term (,10 2 yr) sediment yields from southern Alaska. New low- temperature cooling ages from this area, the glaciated Chugach- St. Elias Mountains, constrain long-term (10 6 yr) exhumation rates. Vertically averaged exhumation rates reach ;3 mm/yr, but are an order of magnitude lower than rates based on short-term sediment yields. Whereas these exhumation rates are not exceptional for oro- genic belts, denudation patterns are strongly correlated with the distribution of glaciers, and erosion appears to keep pace with con- vergence and uplift. These findings imply a coupling between gla- cially dominated erosion and tectonics.

Preservedfoldshapesusuallyreveallittleabouttheirkinematicevolution.Syntectonic strata preserved in growth synclines in contact with a fold, however, can permit ‘‘unfolding’’: a sequential reconstruction of fold growth backward through time from a geometry observed at present to an initial undeformed state. Such reconstructions can define the kinematics of fold growth. Growth strata associated with anticlinal forelimbs in the Ebro basin exhibit depositional tapering of beds on fold flanks and progressive limb rotation. Unfoldingawell-dateddetachmentfolddefinesitskinematicevolutionandcoevallyvarying rates of shortening, forelimb uplift, and forelimb rotation. Interplay of these rates with sedimentation rates controls onlap and offlap relations.

Three independent data sets from northwestern India and Pakistan suggest initial displacement along >1000 km of the Main Boundary thrust prior to 10 Ma, at least 5 m.y. earlier than previously reported. Regionally extensive changes in the depositional characteristics and rates of the foreland-basinfill between 11 and 9.5 Ma are interpreted to reflect new hinterland loading due to the formation of the Main Boundary thrust. Sediment-accumulation rates, sandstone-siltstone ratios, and thickness and amalgamation of individual sandstone bodies all substantially increase after 11 Ma in well-dated stratigraphic sections from Pakistan to Nepal across the Indo-Gangetic foreland basin. In the Himachal Pradesh reentrant of northwestern India, a newly discovered 8.7 Ma conglomerate derived from the hanging wall of the Main Boundary thrust indicates that source-area uplift and denudation must have occurred prior to 9 Ma and probably prior to 10 Ma, assuming a gravelprogradationrateof3cm/yr.Threeapatitefission-trackages fromstructuresattheleadingedgeoftheMainBoundarythrustin the Kohat region of northwest Pakistan indicate that rapid cooling below ;105 C1‐2 m.y. earlier. These data indicate that the Main Boundary thrust in the western Himalaya formed synchronously along strike in the middle-late Miocene, has a displacement rate of ;10 mm/yr, and has a displacement history that is coeval with late displacement on the Main Central thrust.

Sixty-eight 10 Be terrestrial cosmogenic nuclide (TCN) surface exposure ages are presented to define the timing of alluvial fan and strath terrace formation in the hyper-arid San Juan region of the Argentine Precordillera. This region is tectonically active, and numerous fault scarps traverse Quaternary landforms. The three study sites, Marquesado strath complex, Loma Negra alluvial fan and Carpinteria strath complex reveal a history of alluvial fan and strath terrace development over the past ∼225 ka. The Marquesado complex Q3 m surface dates to ∼17 ± 3 ka, whereas the Loma Negra Q1 ln , Q2 ln , Q3 ln , Q4 ln , and Q5 ln surfaces date to ∼24 ± 3 ka, ∼48 ± 2 ka, ∼65 ± 13 ka, ∼105 ± 21 ka, and ∼181 ± 29 ka, respectively. The Carpinteria complex comprises eight surfaces that have been dated and include the Q1 c (∼23 ± 3 ka), Q2 c (∼5 ± 5 ka), Q3a c (∼25 ± 12 ka), Q3b c (∼29 ± 15 ka), Q4 c (∼61 ± 12 ka), Q5 c (∼98 ± 18 ka), Q6 c (∼93 ± 18 ka), and Q7 c (∼212 ± 37 ka). 10 Be TCN depth profile data for the Loma Negra alluvial fan complex and Carpinteria strath terrace complex, as well as OSL ages on some Carpinteria deposits, aid in refining surface ages for comparison with local and global climate proxies, and additionally offer insights into inheritance and erosion rate values for TCNs (∼10 × 10 4 10 Be atoms/g of SiO 2 and ∼5 m Ma −1 , respectively). Comparison with other alluvial fan studies in the region show that less dynamic and older preserved surfaces occur in the Carpinteria and Loma Negra areas with only younger alluvial fan surfaces preserved both to the north and south. These data in combination with that of other studies illustrate broad regional agreement between alluvial fan and strath terrace ages, which suggests that climate is the dominant forcing agent in the timing of terrace formation in this region.